An important development in materials science during the recent past has been the evolution of high performance polymer composites, especially composites with increased strength to weight ratios. One means of obtaining such composites has been the use of fibers as fillers in a polymer matrix to afford fiber-reinforced plastics. In the past the fibers used as a reinforcement have included glass and graphite fiber, and more recently polymer fibers such as polyamide (aramid) fibers have gained increasing use as reinforcing fillers. Still more recently polyethylene and polypropylene fibers have been developed for use in reinforced plastics.
Where used in reinforced plastics, fibers need to have a high tensile strength and a high tensile modulus. When these properties are combined with the lower density of polymer fibers compared with, for example, metals, polymer fibers show a strength to weight ratio perhaps 5-10 times that of metals such as steel and aluminum. Such a weight reduction is important in diverse applications as fabrication of airplane wings, automobile bodies, and golf clubs.
To date high strength polymer fibers have been made mainly from rigid rod polymers. Among the flexible and semiflexible chain polymers, polyethylene, polypropylene and poly(vinyl alcohol) seem to be the only ones from which high strength fibers have beem made to date. In this application we disclose high strength fibers from poly(ethylene oxide), thereby adding to the materials available for use in polymer composites.
The premise underlying the preparation of high strength polymeric fibers is that if long polymer chains in solution can be uncoiled and crystallization induced axially along the linear chains, the resulting filaments will show enhanced tensile strength, ultimately approaching the strength of the weakest bond along the polymer chain. As is usual, the theoretical limits far exceed the reality of practice, and the tensile strength of fibers produced to date are perhaps only 1/10 that theoretically possible, Additionally, however well understood may be the theoretical premise underlying strong fiber production, the practice of obtaining such materials remains unpedictable and subject to constraints not at all well understood. For example, polyethylene filaments can be prepared by spinning a polymer solution, cooling the spun material to a temperature below its gel point, and stretching the resulting filament. Yet the tensile strength is appreciably increased when the polymers have a particular ratio of weight-average to number-average molecular weight. European patent application Publication No. 0077590.
Another method of preparing polyethylene and polypropylene fibers of high tensile strength and tensile modulus involves extrusion of polymer solutions under conditions where the concentration of the polymer in the solution and extrusion are the same, followed by gel formation and stretching of the gel. U.S. Pat. No. 4,413,110. A similar process has been described in U.S. Pat. No. 4,440,711 to make strong poly(vinyl alcohol) fibers.
Crystallization from "stretched" polymer chains may be induced as described by Zwignenburg et al., Colloid and Polymer Science, 256, 729 (1978), where a Couette-type apparatus was used to generate shear and extensional flow fields to afford longitudinal growth of polymeric crystals from a flowing solution. In this method shear and extensional fields are induced by a rotating inner cylinder which uncoils the polymer chains in solution in the annular space, and crystal growth is induced by seed formation either at the rotor surface or at some distance beyond. Strong polyethylene fibers previously have been prepared by this method.